1. Field of the Invention
The present invention relates to an apparatus for detecting the oxygen gas concentration in a sample gas. More particularly, the present invention relates to an apparatus attached to an exhaust tube of an internal combustion engine, which is suitable for detection of the oxygen gas concentration in the exhaust gas, which has a close relation to the air-fuel ratio of an air-fuel mixture supplied to the engine. This oxygen gas concentration-detecting apparatus of the present invention can be preferably used as means for detecting the oxygen gas concentration in an exhaust gas from an external combustion engine or a manufacturing plant. Moreover, the present invention relates to an apparatus for measuring the oxygen gas concentration in an exhaust gas of an internal combustion engine by using the above-mentioned oxygen gas concentration-detecting apparatus and performing feedback control of the air-fuel ratio of the internal combustion engine.
2. Description of the Related Art
As means for detecting the oxygen gas concentration in an exhaust gas of an internal combustion engine (hereinafter referred to as "O.sub.2 sensor"), an apparatus disclosed in Japanese Patent Application Laid-Open Specification No. 203828/84 is known, and the main part for detecting the oxygen gas concentration in the O.sub.2 sensor is known, for example, from Japanese Patent Application Laid-Open Specification No. 204365/83 or Japanese Utility Model Application Laid-Open Specification No. 31054/84.
Namely, the main part of the O.sub.2 sensor has a ceramic tube (ceramic substrate) having the top end portion closed as in a test tube and being composed mainly of zirconium oxide (ZrO.sub.2), and parts of the inner and outer surfaces of the ceramic tube are coated with a platinum (Pt) paste and the ceramic tube is then calcined to form a pair of electrodes for taking out an electromotive force. Furthermore, platinum is vacuum-deposited on the outer surface of the ceramic tube to form an oxidation catalyst layer for oxidizing unburnt components in the exhaust gas, such as CO and HC. Then, a metal oxide such as magnesium spinel is flame-sprayed on the oxidation catalyst layer to form a protecting layer for protecting the oxidation catalyst layer.
In this structure, the air is introduced as the stable reference gas into the cavity of the ceramic tube, and the outer side of the ceramic tube is exposed to an exhaust gas passage of the engine and is contacted with the exhaust gas of the engine. A voltage corresponding to the ratio of the oxygen gas concentration in air contacted with the inner surface of the ceramic tube to the oxygen gas concentration in the exhaust gas contacted with the outer surface of the ceramic tube is generated between the pair of electrodes, and the oxygen gas concentration in the exhaust gas is detected based on this voltage.
It is considered that the electromotive force effect is generated between the electrodes on the inner and outer surfaces of the ceramic tube according to the following mechanism.
If calcia (CaO) or yttria (Y.sub.2 O.sub.3) is added to zirconia (ZrO.sub.2) known as the main component of a ceramic and the mixture is heated, calcia or yttria is included in the crystal and a lattic defect of the oxygen ion is formed, whereby zirconia is formed into a pure oxygen ion conductor in which the oxygen ion moves though either the electron or the hole hardly moves. If the oxygen partial pressure on one wall of densified zirconia is made different from the oxygen partial pressure on the other wall, it is only the oxygen ion O.sup.2- that can move, and as the result, an elecrtomotive force E represented by the following formula is generated according to the oxygen partial pressure ratio: EQU E=(RT/4F)ln(PI/PII)
wherein R stands for the gas constant, T stands for the absolute temperature, F stands for Faraday's constant, ln stands for the natural logarithm, and PI and PII stand for the oxygen partial pressures.
Incidentially, the oxidation catalyst layer of platinum promotes oxidation reactions of CO+1/2O.sub.2 .fwdarw.CO.sub.2 and HC+O.sub.2 .fwdarw.H.sub.2 O+CO.sub.2 between oxygen O.sub.2 and carbon monoxide CO or hydrocarbon HC, and when combustion is carried out with an air-fuel mixture richer than the stoichiometric air-fuel ratio, CO or HC is conveniently reacted with low-concentration O.sub.2 left in the air-fuel mixture to reduce the O.sub.2 concentration almost to zero, whereby the O.sub.2 concentration ratio between the inside and outside of the ceramic tube is increased and a large electromotive force is generated. On the other hand, when combustion is carried out with an air-fuel mixture leaner than stoichiometric air-fuel ratio, since high-concentration O.sub.2 and low-concentration CO and HC are present in the exhaust gas, even if O.sub.2 reacts with CO and HC, O.sub.2 still remains in a considerable amount, and the O.sub.2 concentration ratio between the inside and outside of the ceramic tube is low and no substantial voltage is produced.
Since the value of the electromotive force put out from the O.sub.2 sensor abruptly changes in the vicinity of the stoichiometric air-fuel ratio as pointed out above, by utilizing this phenomenon, it is judged whether or not the air-fuel ratio in an air-fuel mixture sucked in the engine is the stoichiometric air-fuel ratio. If the air-fuel ratio is richer than the stoichiometric air-fuel ratio, the amount of the fuel to be supplied into the engine is decreased or the amount of the intake air is increased, and if the air-fuel mixture is leaner than the stoichiometric air-fuel ratio, the amount of the fuel is increased or the amount of the intake air is decreased. Thus, feedback control of the air-fuel ratio is performed.
Incidentally, in the above-mentioned conventional O.sub.2 sensor, the oxidation catalyst layer has no substantial effect of reducing nitrogen oxides NOx, and therefore, the oxygen concentration in the exhaust gas is detected irrespectively of the concentration of nitrogen oxides NO.sub.2.
Incidentally, nitrogen oxides NOx are formed by bonding of nitrogen N.sub.2 in the air to oxygen O.sub.2 is a high temperature atmosphere.
Namely, O.sub.2 in NOx should be detected as O.sub.2, which has not made any contribution to combustion, for detection of the air-fuel ratio, but this oxygen O.sub.2 is not detected by the conventional O.sub.2 sensor.
Accordingly, the detection value of the O.sub.2 sensor is increased by the amount corresponding to the amount of oxygen which has reacted with nitrogen gas N.sub.2 to form NOx, and in the air-fuel ratio region where the detection value of the O.sub.2 sensor is inverted, the apparent air-fuel ratio is leaner than the actual air-fuel ratio.
Therefore, if feedback control of the air-fuel ratio is performed according to the detection result based on the air-fuel ratio in the inversion region of the O.sub.2 sensor as the reference, the air-fuel ratio is erroneously controlled to a level leaner than the stoichiometric air-fuel ratio as the target air-fuel ratio, and there is a risk that oxidation reaction of nitrogen gas is advanced and nitrogen oxides NOx in the exhaust gas are excessive.
In general, a ternary catalyst for purging the exhaust gas, which is disposed in the exhaust gas passage in the engine, can simultaneously convert CO, HC and NOx efficiently when the air-fuel ratio is close to the stoichiometric air-fuel ratio, but if the air-fuel ratio is controlled to a level leaner than the stoichiometric air-fuel ratio, the conversion of NOx is abruptly reduced and the amount of NOx discharged to the air present downstream of the ternary catalyst passage is drastically increased.
According to the conventional technique, so-called exhaust gas recycle (EGR) control for reducing nitrogen oxides NOx by recycling a part of the exhaust gas of the engine into the intake air and thus lowering the combustion temperature. However, the structure of this EGR control system is complicated because an EGR passage should be laid out and an EGR control valve or the like should be disposed in this passage, and this results in increase of the cost. Moreover, the combustion efficiency is reduced by introduction of the exhaust gas and the fuel expense is greatly increased.
Accordingly, if feedback control of the air-fuel ratio of the internal combustion engine is performed by the conventional inaccurate O.sub.2 sensor, excessive discharge of nitrogen oxides NOx cannot be avoided, and in order to prevent nitrogen oxides NOx from being discharged to the outside, the EGR control system should be disposed in the internal combustion engine, which inevitably resides in the above-mentioned disadvantages.